Internal free magnesium modulates the voltage dependence of contraction and Ca transient in rabbit ventricular myocytes

 We investigated the effect of altering internal free magnesium concentration (Mg_i) on the contraction and Ca_i transient of patch-clamped rabbit ventricular myocytes. Experiments were performed at 35°C; cells were held at –40 mV to inactivate Na channels and T-type Ca channels, and at this potential (and in the absence of cyclic AMP) ”Ca-induced Ca release” is the primary trigger mechanism. Cells dialysed with a low Mg_i (2.9 μM) had a large and fast phasic contraction and Ca_i transient at positive potentials (+60, +80 mV). Cells dialysed with a high Mg_i (7.1 mM) had a small or absent phasic contraction and Ca_i transient at positive potentials. These effects were due to a change in free Mg_i, and not due to a change in [Mg.ATP]. In cells dialysed with a low Mg_i, application of Ca channel blockers (32 μM nifedipine with 10 μM D600) for a single beat abolished current through L-type Ca channels (I _Ca,L); however, 53% of the Ca_i transient was still elicited. Adding 5 mM Ni to Ca channel blockers abolished the remaining Ca_i transient, indicating that (in the absence of I _Ca,L) the transient might be triggered by reverse Na/Ca exchange. In cells dialysed with a high Mg_i, a single-beat switch to Ca channel blockers was sufficient alone to abolish the Ca_i transient, indicating that under these conditions Ca entry via I _Ca,L is the primary sarcoplasmic reticulum trigger mechanism. These results suggest that raised free Mg_i might partially inhibit the activity of the Na/Ca exchange, or might limit its ability to trigger Ca release. Received: 29 August 1997 / Received after revision: 12 November 1997 / Accepted: 13 November 1997